1. Technical Field
This invention relates to an image forming apparatus.
2. Related Art
An electrophotographic image forming apparatus includes an image carrier having a photoconductive layer on which an electrostatic latent image is formed. First, the photoconductive layer is charged and the charge is eliminated in association with image information formed by irradiating the photoconductive layer with a laser, etc., and an electrostatic latent image is formed on the photoconductive layer. This electrostatic latent image is developed using a developing material of toner, etc., and is transferred to a record medium such as a sheet of paper to form an image.
As a system of charging the photoconductive layer, a system of supplying a current from a feeding section through a charging member that comes in contact with the image carrier and charges the photoconductive layer is known. The feeding section supplies a current to the charging member by applying a voltage thereto. The applied voltage is a vibrating voltage with an AC (alternating current) component superposed on a DC (direct current) component (which will be hereinafter referred to as vibration voltage). If only a DC component exists, when the potential difference between the photoconductive layer and the charging member lessens, a further current does not flow and the photoconductive layer cannot be charged to a necessary value. An AC component is superposed, whereby the photoconductive layer is charged to a potential almost equal to the DC component.
When the amplitude of the AC component is small, the charge potential of the photoconductive layer increases in response to an increase in the amplitude. When the charge potential becomes equal to the DC component, if the amplitude of the AC component is further increased, saturation occurs and the charge potential does not change. The boundary between a region where the charge potential increases with an increase in the amplitude of the AC component and a region where the charge potential does not change is referred to as a saturation point. If the amplitude of the AC component is set to the amplitude or more at the saturation point, the photoconductive layer is charged to the necessary potential, as mentioned above. However, although the amplitude is set to the amplitude or more at the saturation point, if it is close to the amplitude at the saturation point, the photoconductive layer is not uniformly charged (charging non-uniformity is occurred) and a minute image defect called a white spot occurs. To suppress occurrence of the image defect, a vibration voltage containing an AC component having amplitude which is sufficiently larger than the amplitude at the saturation point is applied.